We plan to investigate the mechanisms of postreplication repair and recombinational repair in irradiated bacteria and in vitro systems containing an extract of the cells. We will focus on the interaction between damaged DNA molecules and intact covalent circular homologous DNA. Investigations to date have shown that suitable damage in DNA molecules can be produced by crosslinking DNA and allowing the UV endonucleases of the cells to incise the molecules. These damaged molecules appear to initiate genetic exchanges with the undamaged homologs. The frequency of recombination is enhanced and intact covalent circular molecules are cut. We plan to investigate the effectiveness of different types of damaged DNA molecules for initiating this cutting in trans. We will test molecules gapped by incision at pyrimidine dimers and exonuclease-induced gap widening. We will also investigate substrates consisting of molecules containing random single-strand breaks that have been denatured and annealed to produce gaps and other aberrant structures. The effectiveness of these substrates will be tested for both for cutting in trans and for initiating recombination detected by the linking of damaged to undamaged molecules, employing radioactive labels in filter-binding assays. The recA+ protein is required for cutting in trans. We will try to determine what other protein fractions are needed to maintain the stability of covalent circular DNA and permit cutting in trans in the presence of damaged homologs.